Traditionally, photographic films have been used to perform X-ray imaging. Photographic film techniques have the advantages of good spatial resolution (better than 50 μm) and very large active areas. However, use of photographic films suffers from many drawbacks, including low X-ray detection efficiency, non-linearity, and slow image retrieval processes.
Thus, there currently is a growing interest in developing digital radiographic detectors for medical, scientific and industrial applications. The applications for digital radiographic detectors may include medical diagnostic applications, non-destructive evaluation of materials, X-ray diffraction of biological and other material samples, and astronomical observations. For example, some estimates indicate that, in the medical area alone, there are over 600 X-ray images produced per 1000 population per year, much of which may be performed using digital radiographic techniques.
Digital techniques in radiology typically have several benefits over traditional X-ray film analog methods. These include reduced radiation dose for an equivalent image, convenient image acquisition and retrieval (avoiding film development time and cost), digital image processing (image enhancement), computer-assisted diagnosis, and easy image storage and transmission. Furthermore, the ability to provide real time images may be advantageous in some applications.
Recently, amorphous silicon (a-Si:H) transistor-addressed arrays (amorphous silicon arrays) have become a leading technology for large area flat panel imaging. Imagers with up to 2304×3200 pixels (29.2×40.6 cm2) on a single substrate with pitch of 127 μm have been produced, and several companies have started commercial production of the amorphous silicon arrays. Smaller area but higher spatial resolution X-ray imagers are also produced using single crystal silicon CMOS readout technology. The sensitivity to X-rays is obtained by coupling a phosphor screen to either the amorphous silicon array or the CMOS readout. Typically Gd2O2S:Tb phosphor is deposited on the amorphous silicon array-based imagers, although CsI:Tl has also been used.
The detectors utilizing phosphors can be characterized as indirect detectors, which typically require a combination of processes to achieve an image. First, transfer of the X-ray energy into visible light photons by the phosphor should be accomplished, and then subsequently the light should be converted into electrical signals using light sensitive readout arrays.
Although indirect detection may be an improvement over the conventional analog technique using photographic films, this approach may suffer from deficiencies including low efficiency of the energy transfer and limited spatial resolution due to light spreading in the phosphor. The poor energy transfer is due to an inefficient process of creating and collecting visible light photons. The increased light spread is a consequence of increasing phosphor thickness to achieve better efficiency in stopping X-rays. The increased light spread can be ameliorated by use of specially grown CsI scintillators with a columnar structure when the X-rays have low energies and/or the CsI scintillators have thin layers. However, as soon as the aspect ratio (the length of the column to the diameter) increases (e.g., to account for increase in X-ray energies), the light collection within the scintillator columns decreases, further reducing the energy transfer efficiency.
Therefore, it is desirable to provide a digital X-ray detector that can provide efficient energy detection over a wide range of X-ray intensities and improved spatial resolution over phosphor-based digital X-ray detectors.